hamradio
Site Admin
Joined: 03 May 2005
Posts: 46
|
 Posted: Tue Jul 26, 2005 6:14 pm Post subject: HOMEBREWING FOR THE NOVICE |
 |
|
A longing to build a small transmitter, receiver or piece of test equipment is commonly expressed by many amateurs. All too often, however, the longing remains merely that, due to perceived difficulties in obtaining components, a lack of test equipment, or not having a suitable circuit diagram. Yet, these difficulties can be overcome, and the satisfaction of successfully completing a project is immense. This article aims to answer a few of the questions aspiring homebrewers ask.
SELECTING A PROJECT
The first step is to determine what you want out of a project that you are considering starting. Is the device being built for the experience and pleasure that its construction provides, or is it to test a particular circuit technique or component? Maybe the project is because commercially made equivalents are unavailable. Alternatively, it could purely be the satisfaction of working the world with a transmitter you built yourself, or of making measurements with test equipment that would be unaffordable if purchased commercially.
Whatever the reason for building, it is important that the features you want are defined, so that a design can be selected to suit your needs. It may happen that you find a description of a project with all the wanted features, and, furthermore, all parts for it are obtainable. A kit for it could even be available. Otherwise, the constructor may prefer to borrow stages from a range of circuits, ending up with a unique device that meets all requirements. This particularly becomes the case for the more experienced experimenter who seldom builds exactly to published designs.
For the beginner though, it is preferable to work from the one design, and not from parts of several. The decision as to which one depends on available components, features provided, the completeness of the project article, along with complexity and cost considerations.
SOURCES OF INFORMATION
Homebrewers normally have a wealth of material on which to base projects. This is obtainable from:-
Books. There are many publications available to the amateur experimenter. As well as the conventional RSGB and ARRL handbooks, more specialised references cover practical aspects in greater detail. Titles to look out for (whether new or second-hand) include 'Solid State Design for the Radio Amateur', 'QRP Notebook', 'Amateur Radio Techniques', 'G-QRP Club Circuit Handbook', 'WIA Book', and 'Radio Projects for the Amateur', to name a few. Most are obtainable by mail order from Daycom in Melbourne, or via your WIA Divisional Bookshop.
Magazines. In addition to projects in the major amateur periodicals, such as Amateur Radio, Radio and Communications, QST, 73, CQ, Practical Wireless, and RadCom, there occasionally appear radio projects in magazines such as 'Electronics Australia' and 'Silicon Chip'. Some of these designs have the advantage of a kit being available. However, be wary when considering some of the transmitter projects featured; a crystal-locked 100 milliwatt 80 metre AM transmitter is simple and cheap, and may well produce a clean signal on an oscilliscope, but is likely to disappoint when used on air under modern band conditions.
As well as being stocked by the larger newsagents, various local and overseas magazines are carried by public, TAFE and university libraries. These normally provide photocopying facilities, available on a cents per page basis.
In addition, QRP (low power) enthusiasts have their own publications. Probably the best known is 'Sprat', published by the G-QRP Club, renowned for its technical articles and circuit ideas. The US-based QRP Amateur Radio Club International issues 'QRP Quarterly', while the Australian-based CW Operators' QRP Club produces 'Lo-Key'. These magazines widely read by those interested in constructing low-powered transmitters and receivers.
Boomerang Circuit Books. This is a service to members of the CW Operators' QRP Club. These consist of approximately 100 pages of constructional articles, taken from various magazines. These are circulated among participating members. As the only cost involved is the postage of the 'book' to the next member on the list, Travelling Circuit Books are an economical way of gaining access to a wealth of information.
The Internet. With the rapid growth of the Internet, thousands of discussion groups (news groups) have sprung up, catering for a wide range of interests. The one for radio homebrewers is called rec.radio.amateur.homebrew. If you have a particular question, want to know how to obtain a part, or simply want to share your experiences with a particular component or circuit design, the newsgroup would be a good forum for your message. Serving a more specialised audience than newsgroups (such as QRP enthusiasts, for example) are mailing lists. To participate in a mailing list, all you have to do is to send an e-mail message to a particular address saying that you want to subscribe, and before long, you will receive the contents of the list as electronic mail. The Australian QRP Home Page has links to other home pages that feature descriptions of homebrew projects.
TOOLS AND TEST EQUIPMENT REQUIRED
To complete most projects, only basic hand tools will be required.
A 15 to 25 watt soldering iron with a fine tip for electronic work will serve the experimenter well; an expensive soldering station is not needed. Ownership of a reliable multimeter (digital or analogue) is a must, while a general coverage receiver is valuable for checking the operation and stability of VFOs.
A device to indicate the presence of RF is handy for those working with transmitters or receivers. This can be as simple as an RF probe for your multimeter, or could be a stand alone instrument, such as an absorption wavemeter or field strength meter. An RF power meter/dummy load combination is also desirable if building a power amplifier or transmitter.
A dip oscillator is a simple but useful piece of test equipment. It can provide a local signal if aligning a receiver, aid in adjusting antennas, and perform a range of other measurements. Building one is highly recommended, and suitable circuits appear in many of the references cited above.
While a dip oscillator can be used to measure inductance and capacitance, stand-alone instruments provide more accurate results. Some inductance and/or capacitance measurers simply attach to your digital multimeter to provide a reading, while some more expensive multimeters have this circuitry built in as standard. Those continually making tuned circuits, whether for transmitters, receivers or ATUs will find the ability to measure inductance and capacitance indespensible.
A frequently overlooked item of test equipment is the humble portable AM shortwave receiver. Provided the set tunes a large portion of the HF spectrum (say from about 2 to 18 megahertz), it can give a rough indication of a VFO's frequency, verify the operation of crystal oscillator stages, or even act as a crude signal generator. This is made possible by the use of the local oscillator signal generated from within the set; its frequency will normally be 455 kHz below that indicated on the dial.
A frequency counter is a valuable instrument, particularly if you lack a calibrated general coverage receiver. A basic unit covering up to 1 GHz will suffice for most work. Such counters are available off the shelf in ready-built or kit form.
Some form of benchtop power supply is important when testing equipment. Using the standard 13.8 volt, 20 amp supply for experimenting is asking for trouble; most experimental projects require just a few hundred milliamperes, and any accidental short circuits or faulty components could result in spectacular fireworks and further component damage. For this reason, a smaller supply, preferably with short-circuit protection is recommended.
Those experimenting purely with low powered transmitters and receivers will find that 12 volts at one or two amps is all that is required, while other projects incorporating high power linear amplifiers, op-amps or TTL ICs have other requirements. Kits for suitable supplies are readily available, while the use of pre-wired plug packs mean that some can be assembled without performing any 240 volt wiring at all.
CIRCUIT CONSTRUCTION METHODS
While the through-hole printed curcuit board was almost universally used for manufactured electronic equipment (though it is now being replaced by surface mount technology) and kits, there is no reason for the home constructor to use this form of construction for their projects. While conventional circuit boards look neater than some other techniques, they suffer from the disadvantage of requiring a new board to be etched if substantial modifications to the project are desired. Further time is wasted if these do not perform as envisaged. Thus, unless you know the circuit is reliable, it is worthwhile to consider alternatives to the conventional PC boards, particularly if your project uses only discrete components.
One such alternative is to use an etched printed circuit board, but solder components onto the copper side of the board. This obviates the need to drill holes, and makes it easier to make changes. If no ICs are used, there is no need to use chemical resist pens or photographic methods to produce a board; the use of small pieces of adhesive tape placed on the parts of the board where you want the copper to remain will suffice for smaller projects.
The 'paddyboard' form of construction, popularised by Drew Diamond, VK3XU is also suited to smaller projects for which the ultimate in small size is not required. While, like the above methods, it uses PC board material, paddyboard requires no etching; component leads not connected to the copper surface are soldered to small 5x5 mm square insulated pads, made from spare PC board material. These pads may be glued or soldered to the main board. It is very easy to add extra components and even modify circuit layout. Again, paddyboard is most suited to circuits not containing ICs, though this limitation can be overcome if ICs are mounted on small pieces of vero or matrix boards beforehand. The use of high-value resistors (several megohm) as standoff insulators, soldered to the main board is another approach that has worked well. All of the construction methods mentioned so far are suitable for audio, HF, VHF, and perhaps UHF projects.
If compact construction is required, but the constructor is unwilling to use a conventional PC board, matrix board is a good alternative. Having holes punched every 2.5mm, IC projects can be quickly assemled. Matrix board has worked well for RF projects well into the VHF region, and is stocked by the normal parts suppliers.
A refinement of matrix board is veroboard. This is matrix board with a series of parallel copper strips, which can be cut as required by using a drill bit held in the hand. While suitable for power supply and audio projects, the capacitances between the long parallel strips may impair the performance of RF projects. Veroboard can be made into matrix board simply by immersing it in a bath of PCB etchant solution.
APPROACH TO CONSTRUCTION AND TROUBLESHOOTING
Once all components to build a particular project have been gathered, and a construction method has been decided upon, the project can now be assembled. If a simple project, or a well-known design, the entire board can be assembled in the one sitting. Otherwise, if the project is an unfamiliar circuit, or has various stages derived from several sources, it is preferable to build, test, and experiment one stage at a time, before moving on. For this type of construction, where the developmental prototype becomes the final model (possibly after several changes), one can easily see that an adaptable construction method, such as 'Paddyboard' or the use of matrix board is preferable to a PC board, where significant changes require a new board to be etched.
For a large project, such as a large receiver or transceiver, it is desirable that, rather than mounting the entire circuitry on one large board, several smaller boards be used instead. This modular approach to construction permits the project to be an evolving piece of equipment, with additions able to be made as time, inclination and funds permit. This method is also compatible with the 'build and test' approach recommended previously and the desirability of having RF-sensitive stages shielded from one another.
If the project involves RF (especially if it is a transmitter or power amplifier), the box housing it should be shielded. This does not neccesarily mean a conventional metal case is required; boxes made from printed circuit board material is also effective.
The most important aid to trouble-shooting is an ability to think logically. The posession of most of the test equipment mentioned above, plus a schematic diagram of the circuit under test are also desirable. Generally, with trouble-shooting, one checks the overall equipment, by identifying which functions do and do not work, and then attempts to isolate the area of the fault.
In the case of home built equipment under development, it is often not so much a fault, but a performance deficiency that needs to be remedied. This may simply entail the use of a slightly different circuit value for a particular component, or may require the redesign of a whole stage to perform to the specifications required; hence the earlier emphasis on flexible construction methods.
SAFETY
It is not out of place here to discuss electrical safety. The construction of equipment containing high voltages require a change to one's working habits, to minimise the risk of electrocution. The following list, while not exhaustive, shows examples of precautions that should be always taken:-
Do not work on live equipment (switching off is not sufficient - unplug it from the wall)
Discharge electrolytic capacitors before working on a project
Insulate exposed high voltage points in equipment where possible
The current ratings of fuses should be related to the expected current consumption of the project, and not to the contents of your junkbox
Check wiring after construction (preferably by someone other than yourself)
Use proper plugs for power connections
Work with one hand behind your back if you must operate on live equipment
Keep half-built projects and chassis locked away from children
Electrical safety is covered in some of the references mentioned above. In
the meantime, it is wise to steer clear from high voltage projects if you have the slightest doubts about your ability to safely construct them.
CONCLUSION
While the impression may be conveyed that constructing equipment is an activity calling for a high degree of specialised knowledge, and that it is all too hard for the average amateur, nothing can be further from the truth. By starting with simple one and two transistor projects and commercially available kits, one's knowledge will steadily increase to a point where more complex projects can be confidently tackled. By this time, you will be able to construct an item merely from a schematic diagram, and start to develop one's own designs from sections of circuits gleaned from various publications.
USEFUL REFERENCES
Solid State Design for the Radio Amateur
Amateur Radio Techniques
Radio Projects for the Amateur
G-QRP Club Circuit Handbook
WIA Book
ARRL/RSGB Handbooks
PUBLICATIONS FOR THE EXPERIMENTER
QST, AR, Ham Radio, EA, SC, R&C, PW, E&WW, Elector Electronics, Lo-Key, QRP Quarterly, Sprat. |
|
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
